Conventional refrigeration technology typically utilizes a heat pump that relies on compression and expansion of a fluid refrigerant to receive and reject heat in a cyclic manner so as to effect a desired temperature change or i.e. transfer heat energy from one location to another. This cycle can be used to provide e.g., for the receiving of heat from a refrigeration compartment and the rejecting of such heat to the environment or a location that is external to the compartment. Other applications include air conditioning of residential or commercial structures. A variety of different fluid refrigerants have been developed that can be used with the heat pump in such systems.
While improvements have been made to such heat pump systems that rely on the compression of fluid refrigerant, at best such can still only operate at about forty-five percent or less of the maximum theoretical Carnot cycle efficiency. Also, some fluid refrigerants have been discontinued due to environmental concerns. The range of ambient temperatures over which certain refrigerant-based systems can operate may be impractical for certain locations. Other challenges with heat pumps that use a fluid refrigerant exist as well.
Solid state caloric materials (SSCMs), i.e. materials that exhibit a change in entropy in response to an externally applied field, provide a potential alternative to fluid refrigerants for heat pump applications. For example in magnetocaloric materials (MCM), the magnetic moments of an MCM will become more ordered under an increasing, externally applied magnetic field and cause the MCM to generate heat. Conversely, decreasing the externally applied magnetic field will allow the magnetic moments of the MCM to become more disordered and allow the MCM to absorb heat. Some MCMs exhibit the opposite behavior, i.e. generating heat when the magnetic field is removed (which are sometimes referred to as para-magneto-caloric material but both types are referred to collectively herein as magneto-caloric material or MCM). The theoretical percentage of Carnot cycle efficiency achievable for a refrigeration cycle based on an MCM or other SSCM can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. As such, a heat pump system that can effectively use an SSCM would be useful.
Challenges exist to the practical and cost competitive use of an SSCM, however. In addition to the development of suitable SSCMs, equipment that can attractively utilize an SSCM is still needed. Currently proposed equipment may require relatively large and expensive equipment for field application, may be impractical for use in e.g., appliance refrigeration, and may not otherwise operate with enough efficiency to justify capital cost.
Accordingly, a heat pump system that can address certain challenges, such as those identified above, would be useful. Such a heat pump system that can also be used in e.g., a refrigerator appliance would also be useful.